Musannaf Ibn Abi Shayba 1:1329

Abu Bakr narrated to us, he said: Mu'tamir ibn Sulayman narrated to us, from his father, he said: I heard him saying: Ibn Sirin would sometimes spit, then he would say to the man: "Look, has the saliva changed?" If it had changed, he would spit a second time. If it was changed in the third time, he would perform ablution. And if it was not changed in the third time, he did not see [the need for] ablution.