Ahmed was awarded his Test cap by Nasser Hussain at the third and final Test match of England's series in Pakistan before receiving a warm hug from his father, Naeem, who was permitted to join the team huddle.
Rehan, 18 years and 126 days old, had to wait until just the 17th over to bowl his first ball in Test cricket at the Karachi National Stadium.
The teenager had varied his length a bit too much in the first session of play, and went into lunch having bowled five overs for 37 runs, but after lunch was markedly more composed, and claimed his first two wickets in Test cricket, to the delight of his father.
"I was sitting there very quietly just watching him bowl and wanting to enjoy the moment when he took his first wicket," Naeem said.
"I just wanted him to be a bit more relaxed when he bowled and as soon as he relaxed it happened.
"The more relaxed he is then the more the wrist is going to work and the better the ball is going to come out, and when he got the wicket that was the most relaxed he had been all day.
"It was a nice wicket, a really proud moment, and I really enjoyed it. The googly first and then the leg spinner. I had to get out of my seat, I was so happy."
The leg-spinner attracted media attention at just 11 years of age when he dismissed Ben Stokes and Alastair Cook in the nets having been invited to bowl at the national team.
He also received high praise from former Australian spinner Warne, who when he saw him bowling in the nets at Lord's as a youngster said in a video on Twitter: "That's awesome man, really really good, I'm going to keep a close eye on you. I think we're going to be commentating on you really soon.
"I think you'll be playing first-class cricket by the age of 15."
Naeem believes it was evident from when his son was young that he had potential.
"When he was eight or nine years old I could see as a cricketer he had something special," he said.
"The way he trained and the way he was so excited to get to training that he would be ready to go half an hour before we had to.
"He'd be saying 'come on dad, I'm ready to go!' so that kind of thing excited me about him.
"When he bowled to Alastair Cook and the England players, that was when Shane Warne saw him as well and said a few things about him, and that really meant something serious to me.
"It made me think, 'wow, if Shane Warne thinks this, then yes he is going to be something special'."
Rehan received a Test cap from former England captain Hussain with his father permitted to join the team huddle, to his delight.
"That moment, I didn't realise I would be on the ground for it and standing in the huddle with the players.
"I thought they meant for me to be on the side while it happened, but not actually in the huddle, so when they got me in there it was just amazing.
"It is a moment I am never going to forget in my life. It was the best moment I ever had so far and I really enjoyed it.
"With Nasser Hussain as well, a big name, and my heart was beating so fast, but he was really nice and he is a good person."