According to him, it was before the Barcelona vs Manchester United game in the 2011 Champions League final.
"I've been in Arsenal for one day when they allowed us to train to win the Champions League against [Manchester] United [in 2011].
"That's the only time I've been in the Arsenal training centre. Sorry United," Guardiola told reporters before their FA Cup clash against Arsenal.
On the differences between how City and Arsenal play, he added: "They play with wingers higher than the midfielders like us - yeah. But all the methodology, the process, with the character, the mentality, the set pieces, a thousand million things - that belongs to them."