n := int64(123)

fmt.Println(strconv.FormatInt(n, 2)) // 1111011

n := int64(123)
fmt.Printf("%b", n)  // 1111011

x := big.NewInt(123)
s := fmt.Sprintf("%b", x)
// s == "1111011"

/* --- Credit to Dave C in the comments --- */
package main

import (
    "bytes"
    "fmt"
)

func main() {
    fmt.Printf("<%s>n", fmtBits([]byte{0xDE, 0xAD, 0xBE, 0xEF, 0xF0, 0x0D, 0xDE, 0xAD, 0xBE, 0xEF, 0xF0, 0x0D}))

    // OUTPUT:
    // <11011110 10101101 10111110 11101111 11110000 00001101 11011110 10101101 10111110 11101111 11110000 00001101>
}

func fmtBits(data []byte) []byte {
    var buf bytes.Buffer
    for _, b := range data {
        fmt.Fprintf(&buf, "%08b ", b)
    }
    buf.Truncate(buf.Len() - 1) // To remove extra space
    return buf.Bytes()
}

package main

import (
    "fmt"
    "strconv"
    "unsafe"
)

func bInt(n int64) string {
    return strconv.FormatUint(*(*uint64)(unsafe.Pointer(&n)), 2)
}

func main() {
    fmt.Println(bInt(-1))
}