Muwatta Imam Malik Book of Shortening the Prayer, Hadith 77

Yahya related to me from Malik from Nafi that Abdullah ibn Umar used to say, "When a sick man is unable to prostrate he should motion with his head, and not raise anything to his forehead."

وَحَدَّثَنِي عَنْ مَالِكٍ، عَنْ نَافِعٍ، أَنَّ عَبْدَ اللَّهِ بْنَ عُمَرَ، كَانَ يَقُولُ إِذَا لَمْ يَسْتَطِعِ الْمَرِيضُ السُّجُودَ أَوْمَأَ بِرَأْسِهِ إِيمَاءً وَلَمْ يَرْفَعْ إِلَى جَبْهَتِهِ شَيْئًا .