Practically every power statement made about Gohan during the Cell Games, but here's a few of the main, obvious ones.
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Chapter: 403 (DBZ 209), P1.1-3
Context: Goku talking to Cell about Cell's next opponent (ie Gohan)
Goku: “The Cell Games will probably end with this next match. Since if he loses, there won’t be anyone else who could win against you…[ ] But from fighting you just now, I think this guy really can beat you after all."
Chapter: 402 (DBZ 208), P6.5, P7.1-6
Context: after Goku asks Gohan to be the next one to fight Cell
Goku: “In the Room of Spirit and Time, the dormant power sealed away deep, deep inside him began to be released. Why don’t we try asking him? How about it, Gohan? Did you think that dad and Cell’s fight just now was too incredible and you couldn’t keep up?â€
Gohan: “N-no, I didn’t think so…But neither of you were fighting all-out, right…!?â€
Goku: “I don’t know about Cell, but I was going all-out. So in other words, to you it felt like I was holding back?â€
Chapter: 404 (DBZ 210), P2.3-4
Context: after Gohan powers up
Vegeta: “That brat…How did he get such a gigantic battle power?…This is impossible…!â€
Cell: “Looks like what Son Goku said wasn’t a complete bluff…But it seems he overstated things a little when he said you could defeat me…â€</div>