Is it better to offer salah in the beginning of permissible time, or is it the same (in reward) to do at any time during permissible time window

People often say that it is better to offer salat in the beginning of the permissible time, rather than delaying salat till later **in** the permissible time window. Is there Quranic verse or authentic hadith to support this? Another way to ask this question is that in the light of Quran and/or authentic hadith, is salah offered in the beginning of permissible time bracket better than that offered towards the end (but still within) the permissible time bracket, or are both the salahs equal in terms of reward?