Where is that bible verse?

I don't think there's a command as you are wording it in any of the Gospels. However I do think there's something in Matthew's record of the Sermon on the Mount or the Beatitudes that contains something to that effect, although not a command, more like a "blessed are those who defend the weak" sort of thing...without an exact phrase, a concordance is of little help since they don't cross reference ideas.

Psalm 82:3 Defend the cause of the weak and fatherless; maintain the rights of the poor and oppressed.

Psalm 82:4 Rescue the weak and needy; deliver them from the hand of the wicked. NIV

Isaiah 1:16......Stop doing wrong,

Isaiah 1:17 learn to do right! Seek justice,encourage the oppressed. Defend the cause of the fatherless,plead the case of the widow. NIV

Jeremiah 5:27 Like cages full of birds, their houses are full of deceit;they have become rich and powerful

Jeremiah 5:28 and have grown fat and sleek. Their evil deeds have no limit; they do not plead the case of the fatherless to win it, they do not defend the rights of the poor.

Jeremiah 5:29 Should I not punish them for this?” declares the LORD. “Should I not avenge myself on such a nation as this? NIV

Other verses stating a similar subject are throughout God's word.

God Bless

Pro 31:8 ¶ Open thy mouth for the dumb in the cause of all such as are appointed to destruction.

Pro 31:9 Open thy mouth, judge righteously, and plead the cause of the poor and needy.

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NLT

Pro 31:8 Speak up for those who cannot speak for themselves;

ensure justice for those being crushed.

Pro 31:9 Yes, speak up for the poor and helpless,

and see that they get justice.

1.Proverbs 31:8

“·Speak up [L Open your mouth] for those who cannot speak for themselves; defend the rights of ·all those who have nothing [vulnerable/fragile people].

i believe its in the new testament also. however much of hte new testament is quoting lessons from the old testament.

there is definitely some songs that sing how God is strength to the weak.

I think it slipped down between the sofa cushions. Its worth a look. There may be Cheesies down there-or peanuts.