How can a hadith which is disconnected ever be sahih?
Here's the report as it was mentioned in Sunan a-Daraqotni:
حدثنا أبو سعيد الإصطخري , ثنا محمد بن عبد الله بن نوفل , ثنا عبيد بن يعيش , ثنا يونس بن بكير , عن عمرو بن شمر , عن جابر , عن محمد بن علي , قال: «إن لمهدينا آيتين لم تكونا منذ خلق الله السماوات والأرض , ينخسف القمر لأول ليلة من رمضان , وتنكسف الشمس في النصف منه , ولم تكونا منذ خلق الله السماوات والأرض» (Source)
Abu Saeed al-Istakhri told us, Muhammad ibn 'Abdullah ibn Nawfal told us, 'Ubaid ibn Yaish told us, Yunus ibn Bukayr told us, on the authority of 'Amr ibn Shimr, on the authority of Jabir, on the authority of Muhammad ibn 'Ali, he said: “Our Mahdi has two signs that have not occurred since Allah created the heavens and the earth. The moon will be eclipsed on the first night of Ramadan, and the sun will be eclipsed halfway through it, and they have not been since God created the heavens and the earth.”
This report is a fabrication that is falsely attributed to Muhammad ibn 'Ali an-Nafs a-Zakiyya, as such it isn't a hadith, but in best case an Athar since it ends with a tabi'y. The two fabricators and weak narrators of this hadith have already be revealed by the author in a former report, you may find in the book of prayer at the end of a fabrication attributed to 'Aishah () the author's statement on these narrators:
عمرو بن شمر وجابر ضعيفان (Source)
'Amr ibn Shimr and Jabir are weak
And in his a-Do'afaa and al-Matrukoon he also mentioned both.
١٤٠ - جابر بن زيد الجعفي أن اعتبر له بحديث بعد حديث صالح إذا كان عن الأئمة (Source)
140- Jabir ibn Zaid al-Jaafi considered his hadith after a valid hadith if it was on the authority of the imams.
٣٩٧ - عمرو بن شمر أبو عبد الله، كوفي، عن جابر، وعن جعفر بن محمد. (Source)
397 - 'Amr ibn Shimr abu Abdillah, Kufi, on the authority of Jabir, on the authority of Jaafar bin Muhammad.
This alone makes clear that this hadith is weak.
Further, you may find in al-Majruhoon (here and in the next page) of imam ibn Hebban the following words on 'Amr ibn Shimr:
عمرو بن شمر الجعفي
'Amr ibn Shim al-Jaafi.
كنيته أبو عبد الله، يروي عن جابر الجعفي، عداده في أهل الكوفة، روى عنه أهلها، كان رافضيًا يشتم أصحاب رسول الله -صلى الله عليه وسلم- وكان ممن يروي الموضوعات عن الثقات في فضائل أهل البيت عليهم السلام وغيرها، لا يحل كتابة حديثه إلا على جهة التعجب، مات سنة سبع وخمسين ومئة في آخر ولاية أبي جعفر.
His nickname is abu 'Abdillah. He narrates on the authority of Jabir al-Ja’fi. He was counted among the people of Kufa. The people of Kufa narrated on his authority. He was a Rafidite and insulted the companions of the Messenger of Allah - may Allah bless him and grant him peace. He was among those who narrated fabricated stories on the authority of trustworthy people about the virtues of ahl al-Bayt, peace be upon them, and other things. It is not permissible to write his hadith except to express exclamation. He died in the year one hundred and fifty-seven, at the end of Abu Jaafar’s term.
حدثنا مكحول، قال: سمعت جعفر بن أبان، يقول: قلت ليحيى بن معين: عمرو بن شمر؟ قال: ليس بثقة.
Makhoul told us, he said: I heard Jaafar bin Aban saying: I said to Yahya ibn Ma’in: 'Amr bin Shimr? He said: He is not trustworthy.
حدثنا محمد بن إسحاق الثقفي، قال: حدثنا المفضل بن غسان الغلابي، قال: سمعت يحيى بن معين يقول: عمرو بن شمر لا يكتب حديثه.
Muhammad bin Ishaq Al-Thaqafi told us, he said: Al-Mufaddal ibn Ghassan Al-Ghallabi told us, he said: I heard Yahya bin Ma’in say: Amr bin Shimr? do not write his hadith!
And you may find on his teacher in the same book (here and in the following 2 pages) a whole lot of narrations showing his lying and fabrication, here just an extract of his biography:
جابر بن يزيد الجعفي
Jaber ibn Yazid al-Jaafi.
من أهل الكوفة، كنيته أبو يزيد، وقد قيل: أبو محمد، يروي عن عطاء والشعبي، روى عنه الثوري وشعبة، مات سنة ثمان وعشرين ومئة، وكان سبئيًا من أصحاب عبد الله بن سبأ، وكان يقول: إن عليًّا عليه السلام يرجع إلى الدنيا.
From the people of Kufa, his nickname is Abu Yazid, and it has been said: Abu Muhammad. He narrated on the authority of Ata’ and Al-Sha’bi. Al-Thawri and Shu’bah narrated on his authority. He died in the year one hundred and twenty-eight. He was a Saba’i, one of Abdullah ibn Saba's companions, and he used to say: Ali, peace be upon him, would return to this world.
حدثنا إسحاق بن أحمد القطان بتنيس، قال: حدثنا بعباس بن محمد، قال: سمعت يحيى بن معين يقول: جابر الجعفي لا يكتب حديثه ولا كرامة.
Ishaq ibn Ahmad al-Qattan told us in Tanis, he said: Abbas ibn Muhammad told us, he said: I heard Yahya ibn Ma’in say: Jabir al-Ja’fi? do not write down his hadiths and nor of his dignity!
دثنا مكحول، قال: حدثنا أبو أمية، قال: حدثنا أبو سلمة، عن سلام بن مسكين، قال: قال لي جابر الجحفي: عندي خمسون ألف باب من العلم لم أخبر بشيء منه، قال: فذكرت ذلك لأيوب، فقال: أما هو الآن فكذاب.
Makhul told us, he said: Abu Umayyah told us, he said: Abu Salamah told us, on the authority of Salam ibn Maskin, he said: Jabir al-Jahfi said to me: I have fifty thousand chapters of knowledge, but I haven't spread any of it. He said: So I mentioned that to Ayoub, and he said: But now he is a liar.
حدثنا محمد بن سليمان بن فارس، قال: حدثنا محمد بن إسماعيل البخاري، قال: حدثنا الحميدي، قال: سمعت سفيان بن عيينة، يقول: جابر الجعفي يؤمن بالرجعة.
Muhammad bin Suleiman bin Faris told us, he said: Muhammad ibn Ismail al-Bukhari told us, he said: Al-Humaidi told us, he said: I heard Sufyan bin Uyaynah, saying: Jabir Al-Jaafi believes in the return.
حدثنا القطان بالرقة، قال: حدثنا أحمد بن أبي الحواري، قال: سمعت أبا يحيى الحماني، قال: سمعت أبا حنيفة يقول: كما رأيت فيمن لقيت أفضل من عطاء، ولا لقيت فيمن لقيت أكذب من جابر الجعفي، ما أتيته بشيء قط من رأي إلا جاءني فيه بحديت، وزعم أن عنده كذا وكذا ألف حديث عن رسول الله - صلى الله عليه وسلم - لم ينطق بها.
Al-Qattan narrated to us in Raqqa, he said: Ahmad ibn Abi Al-Hawari narrated to us, he said: I heard Abu Yahya Al-Hamani, he said: I heard Abu Hanifa say: As I have seen in someone I have met better than Ata, and I have not met anyone I have met more liar than Jabir Al-Jaafi, I have never presented him with any opinion except He brought me a hadith about it, and claimed that he had such-and-such and a thousand hadiths on the authority of the Messenger of Allah - may Allah bless him and grant him peace - that he did not utter.
After quoting the verdict of imam al-Bukahri and that of abu Hanifa on this narrator ibn Hebban even emphasized abu Hanifa's warning about this man, since abu Hanifa was the leader of the Ra'y school, and explained why such scholars like Shu'aba and Sufyan a-Thawri would note his reports saying (please take further information from the linked biography):
فإن احتج محتج بأن شعبة والثوري رويا [عنه] فإن الثوري ليس من مذهبه ترك الرواية عن الضعفاء، بل كان يؤدي الحديث على ما سمع لأن يرغب الناس في كتابة الأخبار ويطلبوها في المدن والأمصار.
If someone argues that Shu'bah and a-Thawri narrated on his authority, then as for A-Thawri, it is not in his doctrine (madhhab) to abandon narration from the weak narrators. Rather, he used to report the hadith according to what he heard because people wanted to write down the reports and requested it in the cities and regions.
وأما شعبة وغيره من شيوخنا فإنهم رأوا عنده شيئا لم يصبروا عنها وكتبوها، ليعرفوها، فربما ذكر أحدهم عنه الشيء بعد الشيء على جهة التعجب، فتداوله الناس بينهم.
As for Shu'bah and other of our sheikhs, they saw something with him that they could not be patient with, and they wrote it down in order to know it. Perhaps one of them mentioned something after something about him in the form of exclamation, and people circulated it among them.
