Hadith 506742
Chapter 18: Fasting - كتاب الصيام
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
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."
Reference Muwatta Malik 506742
USC-MSA web (English) reference : Book 18, Hadith 43