The hadith was narrated in Musnad Ahmad:
In the first Musnad of the Basri narrators -see here in Arabic-:
In the following I'll translate from Arabic, as these translations are of my own take them with the necessary care! My additions are added in italic
19776 حدثنا محمد بن جعفر حدثنا شعبة عن قتادة عن نصر بن عاصم عن رجل منهم أنه أتى النبي صلى الله عليه وسلم فأسلم على أنه لا يصلي إلا صلاتين فقبل ذلك منه
We have been informed from Muhammad ibn Ja'afar (also known as Ghandar) from Sho'bah (ibn al-Hajjaj) from Qatadah (ibn Di'amah(1)) from Nassr ibn 'Assim from (ibn 'Omar al-Laithi(2)) a man among them that he came to the prophet Allahs prayers and blessings be upon him and embraced Islam with the condition (or making it a condition) not to pray except with two prayers (a day) and it was accepted from him.
(1) The known mufassir and muhadith.
(2) So far all the narrators are from Basra and have a high level of trustworthiness.
In the Musnad of the rest of al-Ansar -see here in Arabic-:
22570 حدثنا وكيع حدثنا شعبة عن قتادة عن نصر بن عاصم الليثي عن رجل منهم أنه أتى النبي صلى الله عليه وسلم فأسلم على أن يصلي صلاتين فقبل منه
We have been informed from Waki'a (ibn al-Jarah)(1) from Sho'bah (ibn al-Hajjaj) from Qatadh (ibn Di'amah) from Nasr ibn 'Assim (ibn 'Omar) al-Laithi (2) from a man among them
that he came to the prophet Allahs prayers and blessings be upon him and embraced Islam making the condition to pray only two prayers (a day) and it was accepted from him.
(1) The known scholar from al-Kufa.
(2) So far all narrators are from 'Iraq, Waki'a وكيع is from Kufa and the others from Basra and all are of a high level of trustworthiness.
In both narrations the final narrator is not named. So far I was not able to clarify whether he is considered as unknown or whether he was named somewhere else or by some later editor or hadith scholar.
In his commentary on Jabir's hadith from Sahih al-Bukhari -see here- ibn Hajar referred to one of the above narrations in Fath al-Bari -see here in Arabic- and added the explanation of Imam Ahmad:
قال الإمام أحمد في رواية ابنه عبد الله : إذا أسلم على أن يصلي صلاتين يقبل منه ، فإذا دخل يؤمر بالصلوات الخمس ، وذكر حديث قتادة عن نصر بن عاصم الذي تقدم .
Imam Ahmad said in the narration of his son 'Abdullah: If he (a person) embraced Islam under the condition to pray only two prayers this is accepted for him. Once he embraces Islam he's (must be) ordered to pray the five prayers. Then he mentioned the hadith of Qatadah from Nasr ibn 'Asim that was mentioned earlier.
In his Nayl al-Awtar -see here- a-Shawkani compiled the hadith among other conditional pledges of allegiance such as this from Sunan abi Dawod saying that these ahadith show that it is allowed to accept the pledges of allegiance and the conversion of (former) disbelievers even if the conditions under which they gave them are void and that accepting Islam by a person who does it with reservation or reluctantly. And he quoted a long quote of al-Khattabi in which he claimed that there are even variations of the hadith quoting one single prayer.