The permissibility of praying behind a seller of qat

Question:
A brother of ours from the Republic of Yemen says: The questioner, Salih Salim Hussein, from Sa'dah Governorate, al-Hashwah district, al-Aqiq. Our brother says that he saw a man whom he believed to hold a higher status than the one he saw him in, which was selling qat. That man is the imam of a mosque, and he asks about the ruling and about praying behind that man?

Answer:
He should be pointed out and advised; because many scholars in Yemen permit qat and consume it, so this imam may have a doubt, he may be one of those who sees it as permissible or follows those who see it as permissible among the scholars of Yemen. So what we see is advising him, and consulting with him to leave it, and not to sell it, and not to drink it. Praying behind him is valid, and praise be to Allah; because the correct opinion among the scholars is that praying behind an open sinner is valid, and it is only invalidated if it is behind a disbeliever. As for the sinner, praying behind him is valid. But it is prescribed for you, my brother, to advise this man or to seek someone who will advise him who is above you, so that he respects him and so that he accepts it from him. And you are rewarded for this, and your prayer is valid, and praise be to Allah.