Muwatta Imam Malik Book of Fasting, Hadith 43

Yahya related to me from Malik that he had heard that Abdullah ibn Umar used to be asked, "an some one fast for some one else, or do the prayer for some one else?" and he would reply, "No one can fast or do the prayer for anyone else."

وَحَدَّثَنِي عَنْ مَالِك أَنَّهُ بَلَغَهُ أَنَّ عَبْدَ اللَّهِ بْنَ عُمَرَ كَانَ يُسْأَلُ هَلْ يَصُومُ أَحَدٌ عَنْ أَحَدٍ أَوْ يُصَلِّي أَحَدٌ عَنْ أَحَدٍ فَيَقُولُ لَا يَصُومُ أَحَدٌ عَنْ أَحَدٍ وَلَا يُصَلِّي أَحَدٌ عَنْ أَحَدٍ