וַיִּמְלֹ֨ךְ שְׁלֹמֹ֧ה בִֽירוּשָׁלִַ֛ם עַל־כָּל־יִשְׂרָאֵ֖ל אַרְבָּעִ֥ים שָׁנָֽה׃
I Salomon królował w Jeruzalemie przez cały Izrael przez czterdzieści lat.
Rashi
And King Rehoboam strengthened himself in Jerusalem And he explains why, because when he was first made king, he was forty-one years old; therefore, he had the strength and the power to strengthen himself. He was a year old when David died, for Solomon reigned forty years, and Solomon had not yet reigned when Rehoboam was born. We find that Rehoboam was forty-one years old when he reigned.