مُوَطَّأُ مَالِكٍ
Muwatta Malik 16
The Times of Prayer
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."
Arabic/English book reference: Book 1, Hadith 16