Nafi' (may Allah have mercy on him) narrates that Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) used to say: When the sick person is unable to prostrate, he should gesture with his head and should not raise anything up to his forehead.
A group likewise narrated it from Nafi', from Ibn Umar, as mawquf (his own words). Abdullah bin Amir al-Aslami narrated it from Nafi' as marfu' (attributed to the Prophet (peace and blessings be upon him)), and that is worthless.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 3671