Who sees you when you stand up.
means, He is taking care of you.
This is like the Ayah,
وَاصْبِرْ لِحُكْمِ رَبِّكَ فَإِنَّكَ بِأَعْيُنِنَا
So wait patiently for the decision of your Lord, for verily, you are under Our Eyes. (52;48)
Ibn Abbas said that the Ayah,
الَّذِي يَرَاكَ حِينَ تَقُومُ
(Who sees you when you stand up) means,
"To pray."
Ikrimah said;
"He sees him when he stands and bows and prostrates."
Al-Hasan said;
الَّذِي يَرَاكَ حِينَ تَقُومُ
(Who sees you when you stand up) means,
"When you pray alone."
Ad-Dahhak said;
الَّذِي يَرَاكَ حِينَ تَقُومُ
(Who sees you when you stand up) means,
"When you are lying in bed and when you are sitting."
Qatadah said;
الَّذِي يَرَاكَ
(Who sees you)
"When you are standing, when you are sitting, and in all other situations."
وَتَقَلُّبَكَ فِي السَّاجِدِينَ