Musannaf Ibn Abi Shayba 3:998

Shababa narrated to us, he said: Layth ibn Sa’d narrated to us, from ‘Uqayl, from Ibn Shihab, that he was asked about a people who prayed on a cloudy day facing other than the Qibla, then the Qibla became clear while they were in the prayer. He said: They should face the Qibla and count what they have prayed. The companions of the Messenger of Allah (may Allah bless him and grant him peace) did that when they were commanded to face the Kaaba while they were in prayer praying towards Bayt al-Maqdis. So they faced the Qibla (the Kaaba) and prayed some of that prayer towards Bayt al-Maqdis and some of it towards the Kaaba.