It was reported that he said about that practice:
Ahmad bin Muhammad bin ‘Eesaa reported in his Nawaadir and Ibn Idrees in his Saraa’ir from ibn Abee ‘Umayr from Hishaam bin al-Hakamm from Abee Abdillaah (this is Ja’far as Saadiq) ( about Mut’ah) and he said: It’s not done with us except by the Fujjaar (transgressors and criminals).
You can see this hadith on-line in volume 100, p 318 of “Bihar al anwar”.
Shia sheikh al-Majad said:
سندها في النوادر معتبر , و قد رواها صاحب الوسائل عن النوادر و سنده الى كتاب النوادر معتبر
“It’s chain in “Nawadir” is reliable (motabar), and it was reported by author of Wasail from “Nawadir”, and his chain till book “Nawadir” is reliable”.
See his fatwa here