"Oto's Invitation" (音の誘い, Oto no Izanai) is episode 109 of the original Naruto anime.
Sasuke briefly battles with the Sound Four members, though they quickly gain the upperhand. After they explain that they were once on Sasuke's level though got stronger once joining Orochimaru, they extend an invitation to Sasuke to accompany them to Orochimaru's lair. After doing so, they leave, allowing Sasuke to consider the matter. The appeal of gaining strength is too much to turn down, and Sasuke makes his way for Konoha's exit. On his way, he meets Sakura, who realises that Sasuke is about to turn his back on the village. She pleads with him to stay, telling him how much she loves him, even offering to help him with his revenge. Unfortunately, she is unable to convince him. Sasuke's cryptic last words before swiftly knocking her out and departing are "Sakura… thank you". He knocks her out then leaves Konoha in the company of the Sound Four.