Passage 1.34.5
δοκῶ δὲ Ἀμφιάραον ὀνειράτων διακρίσει μάλιστα προ ς κεῖσθαι· δῆλος δέ, ἡνίκα ἐνομίσθη θεός, διʼ ὀνειράτων μαντικὴν καταστησάμενος. καὶ πρῶτον μὲν καθήρασθαι νομίζουσιν ὅστις ἦλθεν Ἀμφιαράῳ χρησόμενος· ἔστι δὲ καθάρσιον τῷ θεῷ θύειν, θύουσι δὲ καὶ αὐτῷ καὶ πᾶσιν ὅσοις ἐστὶν ἐπὶ τῷ βωμῷ τὰ ὀνόματα· προεξειργασμένων δὲ τούτων κριὸν θύσαντες καὶ τὸ δέρμα ὑποστρωσάμενοι καθεύδουσιν ἀναμένοντες δήλωσιν ὀνείρατος.
I think Amphiaraus is especially skilled in interpreting dreams; clearly so, since the belief in his divinity was established largely through divination by dreams. The people hold that whoever comes to Amphiaraus seeking an oracle must first undergo purification. It is customary for purification rites to sacrifice to the god, offering not only to Amphiaraus himself but also to all the deities whose names appear on the altar. After these preliminary rites, they sacrifice a ram, spread out its skin, and lie down upon it, sleeping there in expectation of a prophetic dream.