Asalamwalikum. Quite a strange incident happened to m. I woke up for fajr thinking that fajr was at seven when in actual fact it was at quarter to eight. So I made my way to the masjid only to find the door shut. The masjid light was on so I knew he was awake. Now moving on. Confused I asked the imam has prayers finished. He told me clearly namaz is at quarter to eight. Totally disregarding what he said I entered the masjid and prayed my fajr. Only to realise that what the imam said. My question is would I be allowed to pray fajr with jamat even though id prayed? Will my namaz be accepted?
Answer:
In the name of Allah, the most Beneficent, the most Merciful.
If an individual has prayed his fardh salah already and then a congregation starts in the Masjid, then he is only allowed to join the congregation if it is the Zuhr or Isha salah. He will not be allowed to join if it is Fajr, Asr and Maghrib. This is because it is Makruh Tahrimi to pray optional prayers after the Fajr and Asr salah. Also it is not permissible to pray three rakats optional prayer.
In the aforementioned scenario, the Fajr salah you prayed by yourself will be considered as your fardh, therefore, if you were to pray with the congregation that would have been optional which is severely disliked after the Fajr prayer. (Taqrir Abu Dawud p.141 v.2)
Yazid al-Aswad says: “We prayed Fajr prayer with the Prophet of Allah 
at Mina and two men came and stopped at their resting places. The Prophet ordered for them to be brought and they came shaking with fear. The Prophet said to them: ‘what prevented you from praying with the people? …Are you two not Muslims?’ They answered: ‘Certainly we are, O Prophet of Allah, but we had prayed in our resting place.’ The Prophet told them: ‘If you pray in your resting places and then come upon an imam, pray with him, and it will be nafl for you.'” (Sunan Abu Dawud p.92 v.1)
Only Allah Knows Best
Mohammed Tosir Miah
Darul Ifta Birmingham