Our Imam of the Mosque says that Zakat Ul Fitr is wajib on every Muslim who is Sahib-e-Nisab but if He/She is not he/she can pay it. Is it Really the fact?
As far as Zakat ul Fitr is concerned, you can check these Hadith in the given hadiths:
Allah's Messenger (ﷺ) has made Sadaqat-ul-Fitr obligatory, (and it was), either one Sa' of barley or one Sa' of dates (and its payment was obligatory) on young and old people, and on free men as well as on slaves.
It is also to be found in: Hadith About Zakat ul Fitr in Sahih Muslim.
Apart from that, all the Four Imams consider it as Wajib. Below you can see the transcripts from all four schools of thought.
Note: Wajib is also Obligatory, but somewhat lower than Fardh.
In a Fiqh Hanbali Book العدة شرح العمدة it is stated like this.
وهي واجبة على كل مسلم إذا ملك فضلًا عن قوته وقوت عياله ليلة العيد ويومه
And this (Zakatul Fitr) is Wajib on every Muslim, when he has the wealth more than his needs and his children's need on the night and day of Eid (Eid ul Fitr).
In Fiqh Maliki Book الذَّخِيرَة للقرافي it goes like this:
هِيَ وَاجِبَةٌ عِنْدَ مَالِكٍ وَالْأَئِمَّةِ
This (Zakatul Fitr) is Wajib according to (Imam) Malik and other Imams.
In Shafa'i Fiqh Book روضة الطالبين وعمدة المفتين it is stated as:
زَكَاةُ الْفِطْرِ, هِيَ وَاجِبَةٌ
Zakatul Fitr, this is Wajib.
And in Hanafi Fiqh Book الهداية في شرح بداية it goes like this:
صدقة الفطر واجبة على الحر المسلم إذا كان مالكا لمقدار النصاب فاضلا عن مسكنه وثيابه وأثاثه وفرسه وسلاحه وعبيده
Sadaqa tul Fitr (Zakatul Fitr) is Wajib on every Free Muslim, while he is in possession of Nisab, which is extra than his home, clothes, furniture, horses (means of Transport), weapons and Slaves.