I remember somebody already asked for it before and I went through the fic but I didn't saved. I remember Sam was the one who noticed Cas isn't doing well the hunting trips and with time this became a problem in his eyes so he told Dean this and how they did got spanking and learned from that, so he wanted Cas to be more careful too. Dean didn't wanted and couldn't stand spanking Cas, so he leaved while Sam did it. And then Cas got shock, cryed and was shaky on the floor when Dean came back so the elder bro though Sam beated the shit out of Cas... xD
With this details I hope somebody can link it to you,
xxx