We were forbidden from following funeral processions, but it was not made strictly binding upon us.
حدثنا ابن أبي عدي عن ابن عون عن محمد
نهي عن اتباع الجنائز ولم يعزم علينا
Its isnad is sahih. Muhammad is Ibn Sirin. The hadith was narrated by Bukhari 2/99 (Shab edition), Muslim 2/569, Abu Dawud 3/180 no. 3167, and Ibn Majah 1/502 no. 1577, all in the book of funerals, chapter on following funeral processions, and likewise Ibn Abu Shayba 3/285.