The World Federation One Stop Fiqh

Ruling 1038

If a person inadvertently says the dhikr of sajdah before his forehead touches the ground, and before he lifts his head from sajdah he realises he has made a mistake, he must remain still and say the dhikr again. However, if his forehead touches the ground and he inadvertently says the dhikr before his body is still, it is unnecessary to repeat the dhikr.