Berachos, Chapter 2, Mishnah 5. After dealing with the matter of concentration during Kerias Shema, the Mishnah teaches us a further law that stems from that same requirement of concentration: the law of a bridegroom on the first night of his marriage.
The words of the Mishnah:
"Chasan patur mi'kerias Shema ba'lailah ha'rishon ad motzaei Shabbos, im lo asah maaseh" - a bridegroom is exempt from Kerias Shema on the first night until the conclusion of Shabbos, if he has not performed the act. Let us explain the details of this law:
"Chasan" - one who has married a woman and has not yet consummated the marriage with her, which is itself the performance of a mitzvah.
"Patur mi'kerias Shema" - since he is occupied with a mitzvah and preoccupied with it, his mind is distracted, and he is not considered capable of concentrating during Kerias Shema, which requires concentration.
"Ad motzaei Shabbos" - according to the accepted practice in the days of Chazal, when people would marry on Wednesday, so that the exemption lasts four days.
"Im lo asah maaseh" - all this applies when he has not yet consummated the marriage.
But once four days have passed, even if he has not consummated the marriage, we already assume that he is capable of directing his mind during Kerias Shema.
The incident with Rabban Gamliel:
The Mishnah brings an incident involving this law: "Maaseh b'Rabban Gamliel she'kara ba'lailah ha'rishon she'nasa" - Rabban Gamliel recited Kerias Shema on the first night of his marriage, even though it is clear that he had not yet consummated the marriage. His students said to him: "Did you not teach us, our teacher?" - you taught us the very law that we learned in the Mishnah, that a bridegroom is exempt from Kerias Shema on the first night. He said to them: "Eini shomea lachem l'vatel mimeni malchus Shamayim afilu shaah achas" - I will not listen to you, to remove from myself the kingship of Heaven even for a single hour.
The essence of Kerias Shema is the acceptance of the kingship of the Holy One, Blessed is He, and about this Rabban Gamliel said that he was not prepared to remove himself from the idea of the kingship of Heaven even for one hour. Therefore, even though he was a bridegroom, it was incumbent upon him to be capable of fulfilling his obligation of accepting the yoke of the kingship of Heaven. Rabban Gamliel thus acted stringently with himself even though by law he was exempt, and this was because of the level at which he conducted himself in accepting the yoke of the kingship of Heaven.
In summary: in this Mishnah we learned that a bridegroom is exempt from Kerias Shema on the first night and until the conclusion of Shabbos, as long as he has not consummated the marriage, because he is occupied with a mitzvah and its preoccupation distracts his mind from the required concentration. We also learned from the incident with Rabban Gamliel, who recited Kerias Shema on the first night and was stringent with himself despite the exemption, because he was not prepared to have the kingship of Heaven removed from him even for a single hour.