Language note: Note the use of 'whom' after a preposition. If you're not sure when to use 'who' and 'whom', see a useful rule of thumb and then do a quiz.
Additional language note: Many grammar books will tell you that 'Whom did you see?' is the correct form as 'whom' is the direct object. However, in spoken English 'Who did you see?' is far more common and 'Whom did you see?' could even be regarded as posh or pretentious. It's what the Queen would say, for instance. Having said that, in exams (TOEIC, TOEFL etc), you'd better go with the 'correct' form.