Muwatta Malik 16

وَحَدَّثَنِي عَنْ مَالِكٍ، عَنْ نَافِعٍ، أَنَّ عَبْدَ اللَّهِ بْنَ عُمَرَ بْنِ الْخَطَّابِ، كَانَ يَقُولُ إِذَا فَاتَتْكَ الرَّكْعَةُ فَقَدْ فَاتَتْكَ السَّجْدَةُ ‏.‏

Yahya related to me from Malik from Nafi that Abdullah ibn Umar ibn al-Khattab used to say, "If the ruku has passed you by, so has the sajda

Reference

Muwatta Malik 16

In-book

Book 1, Hadith 16