Muwatta Malik 530

وَحَدَّثَنِي عَنْ مَالِكٍ، عَنِ ابْنِ شِهَابٍ، أَنَّهُ قَالَ الْمَشْىُ خَلْفَ الْجَنَازَةِ مِنْ خَطَإِ السُّنَّةِ ‏.‏

Yahya related to me from Malik that Ibn Shihab said, "Walking behind the bier is in contradiction to the sunna