Is it permissible for someone who has eaten onions to pray in congregation with his brothers, or should each of them pray alone?

Eating garlic and onions is permissible, though disliked. It is better to avoid eating them before prayer. Whoever eats them incurs no sin, even if they miss congregational prayer in the mosque, unless it is the Friday prayer, in which case it is not permissible to intentionally eat them after the call to prayer.

If an entire group has eaten onions, they may pray together in their home. Sheikh Al-Uthaymeen says: It is not disliked for a person to pray in congregation after eating onions or garlic, unless it harms the worshippers, in which case he should not join them.

If the smell of garlic or onions dissipates over time or by using something that removes the odor, the reason preventing attendance at the mosque is gone, and there is no justification for missing congregational prayer, based on the saying of Umar ibn Al-Khattab, may Allah be pleased with him: "O people, you eat two plants which I do not consider anything but foul: this garlic and this onion. I saw a man during the time of the Messenger of Allah, peace and blessings be upon him, whose odor was detected from him, and he was taken by the hand until he was led out to Al-Baqi'. So, whoever must eat them, let him cook them until their odor is gone."