in the morning and in the evening
بكرة وأصيلاً
This is a specific mention following a general one, both in the act and in the time. As for the act, tasbih is a type of dhikr, so it is a specification after a generalization. As for the time, here it is specified as the morning and the evening, whereas the dhikr itself is left general. This verse is like His saying, the Exalted:
and exalt [Allah] with praise of your Lord before the rising of the sun and before its setting
وَسَبِّحْ بِحَمْدِ رَبِّكَ قَبْلَ طُلُوعِ الشَّمْسِ وَقَبْلَ غُرُوبِهَا
[Ta-Ha:130] He says:
remember Allah with much remembrance (42) And exalt Him
اذْكُرُوا اللَّهَ ذِكْرًا كَثِيرًا * وَسَبِّحُوهُ [Al-Ahzab 33:41-42]
The meaning of tasbih is declaring Allah free from every defect and flaw. Among the flaws is resembling the created beings or being like the created beings. So when you say “Subhan Allah,” the meaning is that you declare Allah free from every defect and flaw, and among the flaws is being like the created beings. He is free from being like the created beings and from every defect in His attributes. He is All-Hearing, free from any defect in hearing; He is All-Knowing, free from any defect in knowledge, and so on with the rest of the attributes. And His saying:
glorify Him in the morning
سبحوه بكرة
The author (may Allah have mercy on him) said:
“The beginning of the day and its end” meaning that the morning is the beginning of the day and the evening is the end of the day. Accordingly, Allah, Mighty and Majestic, has commanded us in the morning and the evening.