Musannaf Ibn Abi Shaybah 3409 — A man praying part of his prayer

Uqayl said: Ibn Shihab was asked: "If, on a cloudy day, people pray facing a direction other than the qiblah, and while in prayer it becomes clear that the qiblah is in another direction, what should they do?" He said: "They should turn toward the qiblah, and they need not repeat what they have already prayed. The Companions of the Prophet (peace and blessings be upon him) did the same when they were commanded to make the Ka'bah their qiblah. Before that they were praying facing Bayt al-Maqdis; after this command they turned toward the Ka'bah. Thus they prayed part of the prayer facing Bayt al-Maqdis and part of it facing the Ka'bah."