From the Prophet ﷺ. I heard Sufyan say, "Beware of suspicion, for it is the most untruthful of speech."
قال قرئ على سفيان سمعت أبا الزناد عن الأعرج
عن النبي ﷺ فسمعت سفيان يقول إياكم والظن فإنه أكذب الحديث
Its isnad is sahih. This is the beginning of a long hadith, which Malik related in Muwatta 907-908, from Abu Zinad. It will come again by way of Malik at 10002. It also comes by other routes, including 7845, 8103, 8485. Bukhari related it at 10:404, and Muslim at 2:279, both by way of Malik, at length. Bukhari also related it at length by other routes, at 9:171 and 10:401-403, and 4:123. Ahmad's statement here, "I heard Sufyan say," and so on, means that the isnad of the hadith was read aloud to Sufyan, and then Sufyan recited the main text.