What's the ruling on praying behind an Imam who prostrates while his feet are dangling in the air because he is ignorant that it is a pillar to prostrate on seven bones including the feet .are my prayers offered behind such an Imam valid or invalid.

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

A Muslim should not raise his feet from the ground during prostration because that is contrary to the guidance of the Prophet as it is proven that he said: “I have been commanded to prostrate on seven bones: the forehead, the hands, the knees, and the toes of both feet.” [Al-Bukhari and Muslim]

If the worshiper raises his legs after putting them on the ground, or returns them onto the ground after lifting them, then his prayer is not invalidated, Allah willing, because the prostration took place by placing them on the ground, but he should not raise them as that is contrary to the Sunnah, as stated above.

In case he did not prostrate on them at all, then the scholars differed in opinion regarding the ruling on his prayer, and perhaps the most correct of their sayings, Allah willing, is that it is valid; this is the view of the Maaliki School of jurisprudence and those who share the same view with them.

In any case, your prayer behind this Imam is valid but you should advise him and counsel him not to raise his feet during prostration without an excuse.

Allah knows best.